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Transparent polycrystalline cubic spinels protect and defend Figure 6. Performance of γ-AlON (upper) and glass (lower) in a simulated rock strike test. The glass tiles experience severe cracking, whereas the γ-AlON remains intact. ics are being evaluated. Designs for an electro-optic defense system under development, the Common Infrared Counter-Measures, also may include mid-IR transparent ceramic hyperhemispherical domes. • Reconnaissance and sensor windows Reconnaissance systems with imaging capabilities for surveying and sensing field conditions, such as terrain or heat, have stringent optical requirements. A typical specification for transmitted wavefront uniformity allows less than one-tenth of a wavelength of error over the size of the sensor aperture. Some helicopter-based sensors and aircraft-based targeting pods now have γ-AlON windows installed. • Night vision systems The current technology NV systems, Generation III, sense signals over wavelength range of 0.4–0.92 micrometers. Transparent γ-AlON-based armor has a high transmission over this wavelength range and has shown 40–50 percent improvement in NV transmittance performance compared with glass armor designed for the same ballistic threat (Figure 2). NV performance is expected to be increasingly important for next-generation systems. • GRIN optics This new and highly advanced application area for transparent ceramics is still in the development stage. This technology reduces the number, weight, and complexity of optical train components in military systems, such as image systems for laser range finders, NV goggles, and unmanned aerial vehicles. • Windows for laser communications Transparent windows or domes protect laser systems from the outside world for many airborne laser-based systems, such as (Credit: Surmet.) laser data links, Counter Manpads (shoulder launched missiles), laser rangefinders, laser target designators, and laser radars. Military optics systems must meet the optical specification for transmitting laser light with high efficiency, low absorption and scatter, and minimal distortion. The extremely high Table 3. Other useful properties of γ-AlON and magnesium-spinel Property Description and potential application areas rf transparency γ-AlON is transparent to radio frequencies, which is useful for selective rf communications and microwave related applications. γ-AlON has a high dielectric constant (>9), low loss tangent, and high Dielectric constant and break-down strength. Its use as a bulk ceramic in the electronic and strength semiconductor industry has been very limited. However, amorphous γ-AlON coatings are used as dielectrics because of their unique combina- tion of mechanical and electrical properties.9 Laser windows transmit laser beams efficiently while protecting the laser from the outside world. The thermal shock resistance, hardness, and Laser damage threshold strength, as well as high optical quality (low absorption, low scatter, and minimal distortion) of γ-AlON make it suitable for as laser windows. Ability to dope, optical transparency, and low phonon energy (low nonradiative transition) make γ-AlON an excellent host material for Upconversion phosphors. Several studies reported upconversion luminescence behavior phosphorescence when doped with rare-earth ions. Broad emissions in the visible wave- lengths were recorded under UV excitation. Similar behavior also was reported for Mg-spinel.10 Mg-spinel is a potential scintillator host for γ-ray detection for Radiation detection via medical imaging. When doped with cerium, Mg-spinel shows promising scintillation luminescence behavior and better optical transparency compared with materials such as LaBr3:Ce and LaCl3:Ce.11 24 www.ceramics.org | American Ceramic Society Bulletin, Vol. 92, No. 2


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